It is known to impart vibrational energy to a test piece to evaluate the vibration resistance of the piece, design defects and manufacturing defects. Many electronic, mechanical, and electromechanical products, particularly those intended for aerospace applications are thus screened before use. Routine quality control inspection of products having electronic, mechanical, and electromechanical components by vibration evaluation is less common, although such testing will reveal component problems and misassemblies, and other manufacturing defects in products having good design but poor manufacturing execution. Such quality control inspection as an integral part of the manufacturing process is referred to herein as product "screening" in that defective products are screened from other production.
In general, vibration shakers used for design evaluation and destructive testing are too complex, too large and cumbersome to operate for production use by manufacturing companies. What has been available in shakers has been largely aimed at test laboratories and has had features and capabilities, and prices, commensurate with the needs and capacities of that market.
There exists a need for a low cost, relatively compact screening vibration apparatus which has the features important to screening vibration use, e.g. easy operation, high reliability, precision control and a vibration capability likely to reveal manufacturing defects in electronic, mechanical and electromechanical products. Ideally, such apparatus will vibrate the test piece in two axes simultanously, for thorough inspection in minimum time.
Simultaneous two axis vibration of a common slip plate on which the test piece is carried requires vibration energy input in two different axes, e.g. at right angles to each other, with the first vibration input not limiting the second input, despite being coupled to a common slip plate. Also, the tendency of the inputs to deteriorate into movement around rather than across the slip plate center vertical axis, i.e. pivotal or rotational movement about that axis of the slip plate must be blocked if true two-axis linear vibrational movement is to be realized. These seemingly inherently contradictory requirements have been previously unresolved, with attempts involving mechanical coupling of the slip plate to the shaker through spherical or swivel couplings at either end of coupling links, but in the present invention method and apparatus are provided which affords true two-axis vibration, prevents pivoting motion, and does so in a simple, straightforward manner so that the apparatus is readily constructed, easily operated, low in capital cost, and precisely reliable in screening testing applications. Unlike, the swivel coupling devices, the present apparatus avoids the addition of mass inherent in using links, avoids the use of high pressure hydraulic systems to keep the swivel couplings lubricated, and avoids driving through an angle which is characteristic of rigid link systems.